1. Technical Field
An elevator brake that utilizes the elevator platform weight to generate the clamping force for stopping the elevator is disclosed. The gravity-assisted elevator brake can constitute a safety system for ropeless or non-trunked elevators in which the conventional overspeed governor-based safety systems can not be used. The gravity-assisted elevator brake unit that can be released and applied with minimal force so it does not require a large power supply.
2. Prior Art
In a ropeless or non-trunked elevator platform system, there are no ropes; the lift force is often from a linear motor. Conventional overspeed governor-based safety systems can not be used in this case. U.S. Pat. No. 5,234,079 disclosed a ropeless linear motor elevator system comprising a brake unit and an electromagnetic brake. The brake systems include brake shoes and mechanisms to engage the brake shoes with the rails to stop the platform. The brake unit as a parking brake is mounted between the drive unit and the platform and it consists of a pair of brake shoes mounted on the elevator platform with a mechanical actuator to apply and release the brake. The electromagnetic brake is mounted below the platform and it also consists of a pair of brake shoes mounted on the elevator platform with an electromechanical actuator to apply and release the brake. In the brake unit and the electromechanical brake, the forces that clamp the brake shoes against the rails are generated by the set of springs. As the weight of the platform increases, the clamping forces and the size of the springs have to be appropriately increased. Increasing the-spring forces will require increasing power of the actuators that apply to release the brake during operation.
A safety disc brake system for lifts that is released electromagnetically was described in U.S. Pat. No. 5,253,738. U.S. Pat. No. 5,518,087 presented a rail brake apparatus for a linear motor elevator. This invention is an improvement over the disc safety brake system in U.S. Pat. No. 5,253,738 and the electromagnetic brake with clamping jaws that was described in U.S. Pat. No. 5,014,828. Even with these improvements, a large force is still needed to operate the brake. In these patents the clamping forces and hence the power requirements for the electromagnets are not disclosed. It is unlikely that a ropeless elevator platform will tethered to a power cable, so supplying large amounts of power to the safety systems could be a problem.
Electromagnetically released safety systems that have been considered for ropeless linear motor elevator systems rely on resilient or spring members to generate the clamping force. As the weight of the platform increases, the clamping forces and the size of the springs have to be appropriately increased. The force needed to counteract the spring-preset force must be greater than clamping force.
The object of this invention to provide a ropeless elevator a safety system that utilizes the weight of the elevator platform to generate large and necessary clamping force thus minimizing the power required operating the brake.
Another object of this invention is to develop a brake system that can be a parking brake and an emergency brake when the lift system fails or when power is lost while necessary power to keep the braking pads ajar from the rail is minimal and independent from the weight of the platform and cargo.
Another important object of this invention is to develop a brake system in which clamping force is self adjusted according to the total weight of cargo and elevator.
This invention is to provide a safety system for high capacity ropeless elevator platform. In developing brakes for high capacity platforms, generating the large clamping forces with limited power sources on the platform is a major problem. To illustrate the problem, consider the motion of the platform during braking. The variables are as follows:
M mass of platform and cargo.
B Drag on the platform, assumed to be zero
FB Braking Force
Vint Initial platform speed before the brake is applied (assumed going down)
W Weight of the platform and cargo
ts Stopping time after the brake is applied
It follows from linear motion analysis that the brake force FB needed to stop the platform in ts, is given by:
FB=W+M Vint/ts.xe2x80x83xe2x80x83(1)
The term (M Vint/ts) represents the change in momentum. The braking force FB is due to a clamping force FG. The clamping force FG is often due to a braking unit reacting against a rail. The braking force is given by
FB=xcexcFGxe2x80x83xe2x80x83(2)
xcexc is the coefficient of friction, in this case this will be the coefficient of friction between the clamping rollers or pads and the rails.
Hence the clamping force FG, is given by:
FG=(W+M Vint/ts.)/xcexc.xe2x80x83xe2x80x83(3)
So the clamping force without gravity assist has to exceed the total weight of the platform and cargo by a factor of 1/xcexc, even if the momentum term is neglected.
Consider a ropeless platform on a ship. For completeness, assume that the platform weighs 6000 lbf with a load of 12,000 lbf. In relatively high sea states, the effective weight of the platform, is approximately 27,000 lbf. Assuming a xcexc=0.5, the clamping force has to exceed 54,000 lbf. Given the initial speed of the platform and the reaction time of the braking system, a more accurate estimate of the clamping force can be calculated using equation (3). To develop clamping forces using springs, the actuator for releasing the brake even with a mechanical advantage of 10, will still have to apply well over 5000 lbf. In the gravity-assisted elevator brake the clamping force is developed from the elevator platform and load weight. The weight is magnified by the mechanical advantage of the lever arm.